Holding tool........I put a big flat screw driver between a ring gear tooth and that little square block on the side of the cylinder. Made a helix tool but my buddy made the threads for in the jack shaft.
If you push the inner half all the way in the bolt head can hit if they are turned the right way. I don't think the secondary shifts all that way out by the look of the belt marks on the sheave.
Around that jet size there is different flowing jets even if the numbers get bigger the engine could run lean. There are 520 and 520 n. 520 could be leaner than 500. Do some searching before you go any further. I thought 490 was ok, but after 500 thing went different. caution!